Advancements in propulsion technologies have required gas turbine engines to operate at higher temperatures. This increase in operating temperature has required concomitant advancements in the operating temperatures of metal (e.g. superalloy) turbine engine components. Thermal barrier coatings have been used to meet these higher temperature requirements. Typical thermal barrier coatings comprise alumina and/or zirconia based ceramic which provide a thermal insulative layer to protect the metal component from the high temperatures.
Thermal barrier coatings have been applied to metal components by first coating the component with a bondcoat, which may comprise an inwardly or outwardly grown platinum modified diffusion aluminide bondcoat and/or MCrAlY overlay bondcoat where M is Ni and/or Co. After applying the bondcoat, the coated component typically is grit blasted and vacuum heat treated, or vice versa, to promote the formation of a thermally grown oxide (TGO) layer typically comprising alumina on the aluminum-rich underlying bondcoat. The component then is coated by electron beam physical vapor deposition with a thermal insulative layer of alumina, zirconia, or other ceramic material. For example, U.S. Pat. Nos. 5,716,720 and 5,856,027 describe a thermal barrier coating system comprising a clean platinum modified diffusion aluminide bondcoat on the substrate, a thermal grown alumina layer, and a thermal insulative ceramic layer on the alumina layer. The platinum modified diffusion aluminide bondcoat comprises an outwardly grown diffusion aluminide coating produced by CVD (chemical vapor deposition) processing using high substrate temperature and low activity coating gases that produce higher concentrations of Pt and Al and low concentrations of harmful refractory metal impurities (e.g. Mo, W, Cr, Ta, etc.) and surface active impurities (e.g. S, P, Cl, B, etc.) at the outermost zone or region of the aluminide coating.
The life of a thermal barrier coating; i.e. time to coating spallation, is known to be related to the surface characteristics of the bondcoat and the particular phase of the thermally grown alumina present between the insulative layer and the bondcoat. Negative effects of bondcoat surface roughness on coating life have been reported by Jordan in “Bondcoat Strength and Stress Measurements in Thermal Barrier Coatings”, US Department of Energy Report (subcontract # 95-01-SR030), Sep. 30, 1997, where it was reported that platinum aluminide bondcoat surfaces include grain boundary ridges that act as sites for stress concentration and damage accumulation during thermal cycling of thermal barrier coated substrates. Grain boundary ridges act as sites for preferential oxidation, void formation, and crack initiation which result in the thermally grown alumina spalling prematurely.
The formation of the thermally grown alumina layer on the aluminum-rich bondcoat involves several metastable transition phases, such as a cubic gamma alumina phase transforming to a tetragonal delta alumina phase then to a monoclinic theta alumina phase finally to a rhombohedral alpha alumina phase, the formation of which occurs by heterogeneous nucleation and growth of the alpha phase from the monoclinic theta phase. The sum of the metastable transitions involves a substantial molar volume reduction of approximately 9%, a significant portion of which is attributable to the final transition from the theta phase to alpha phase.
An object of the present invention is to provide a method of pretreating a superalloy or other substrate prior to coating with a thermal insulative layer of a thermal barrier coating system in a manner to reduce adverse effects of bondcoat surface roughness on life of the thermal barrier coating.
Another object of the present invention is to provide a method of pretreating a superalloy or other substrate prior to coating with a thermal insulative layer of a thermal barrier coating system in a manner to reduce adverse effects of metastable phases of thermally grown alumina on life of the thermal barrier coating.
A still further object of the present invention is to significantly increase the life of the thermal barrier coating system under high temperature cyclic oxidation conditions.